With the advent of the use of glass fiber filaments for transmission of signals by means of light pulses, the assembly of the fibre in a cable presented problems not encountered in the cable industry dealing with cabling of metallic strands.
The glass fibre has unique properties. The fibre is thin, in the order of 2-3 mils in diameter. It has high tensile strength of the order of 150,000-200,000 PSI, that is 1.5-2 lbs. per 3 mil diameter filament. However, there are spots in a kilometer length of a fibre, which will sustain only 10% of the ultimate strength, that is 0.2 lbs. The glass fibre is perfectly elastic up to about 1% elongation, after which the fibre snaps.
To preserve the glass fibre from snapping, when subject to tension, the fibre should not be subject to tension greater than 10% of the strength at break in consideration of this weak spot. At 10% of the ultimate strength there will be also 10% of ultimate elongation, that is 0.1%. Various schemes are proposed to combine a number of glass fibres, each individually coated with one or more layers of plastic, to protect the glass from abrasion, moisture and against excessive bending. The coated glass fibre has a diameter of 6-15 mils. One scheme of cable making follows the conventional cable making practice. It is to strand the fibres around a supporting member of steel or plastic. Another is to reinforce the optical fibres by cabling them with non-optical glass fibres for reinforcement, so that the fibre can withstand tension inherent in cable handling and installation.
The common shortcoming of all those schemes is lack of realization that any stress-supporting or reinforcing member, be it the coating of the glass fibre by plastics or a strength member placed inside or outside of the bundle of optical fibres, will not provide needed tensile force, until it is stressed, at which it will elongate. For instance, conventional electrical cables, at installation, or wrapped around cable drum, may elongate by 1-2-3% with the elongation retracting when pulling force is removed. However, glass fibre, as stated above, will snap already at 0.1% elongation of cable. This fact makes conventional cable making concepts unsuitable for making of cables out of optical glass fibres.
The subject of this invention, therefore, is a cable in which a strength carrying member may elongate by 2-3% while the glass fibre is not stressed and is not strained even the 0.1%, theoretically permissible. To achieve this effect a special construction is provided with helically wrapped optic fibres supported from a strength carrying member consisting of a tube, and with the fibres supported so as to permit decrease in the diameter of the helical convolutions when the strength carrying tube is elongated, and without tensioning the optic fibres to their elastic limit.
Other objects, features and advantages of the invention will appear or be pointed out as the specification proceeds.